Radio receiver tone control circuit



Oct. 3l, 1944. E. l.. CLARK RADLOA RECEIVER TONE CONTROL CIRCUIT Filed July 18, 1942 Patented Oct.- 31, 1944 RADIO RECEIVER TONE CONTROL CIRCUIT Edwin L. Clark, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 18, 1942, Serial No. 451,427

(Cl. Z50-20) 9 Claims.

My present invention relates to radio receivers, and more particularly to an automatic tone control circuit for a radio receiving system.

One of the main objects of my invention is to 'provide in the audio frequency transmission network of a radio receiver, an automatic control circuit which functions to ladjust the higher audio frequency response of the audio network in accordance with changes in the strength of received signals.

Another important object of the invention may .l be stated to reside in the provision of a tone control circuit for an audio frequency transmission network, and wherein the tone control circuit comprises a network for degenerating the higher audio frequencies of the audio energy.

Another important object of the invention is to provide a radio receiver employing a demodulator followed by an audio amplifier network;

a phase inverter stage is employed in order to feed a push-pull audio amplifier from a singleended audio stage, and there being provided an audio path between the phase inverter and the single-ended audio stage which is degenerative for the higher audio frequencies, thereby to provide a means for controlling the audio response of the receiver.

Other objects of my invention are to provide an automatic tone control for an audio amplifier which permits a wide degree of tone control action without distortion; to provide a type of tone control which can be used for manual operation and the control being vactuatable from a remote point; to provide a type of tone control which permits reception of fading short wave signals itself, however, as to both itsorganization and method of operation will best be understood by reference to the following' description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect. 55

Referring to the accompanying drawing, I have shown so much of a radio receiving system as is essential to a proper understanding of this invention. It is to be understood that the receiver may be a superheterodyne of the multi-wave typ'e. Such a receiver is well known to those skilled in the art, and usually embodies a wave-changing device for rendering the receiver operative in the standard broadcast band or in any of the short wave bands used in this country or in foreign countries. Regardless of the band in which the receiver is operating, the construction of the receiver` subsequent to the converter will be the same for al1 wave bands. For the sake of illustration, let it be assumed that the receiver has been adjusted to operate in theshort wave band.

In such a band the eifect of selective fading is a problem which is generally encountered. With selective fading the carrier fades with respect to its modulation sideband components, and there is produced a type of noise which is highlyundesirable. My present invention, which comprises the automatic audio tone control mechanism to be hereinafter disclosed, is particularly advantageous in overcoming `the noise effects produced during such periods of selective fading..

It is to be particularly understood, however, that the invention is not limited to the short wave band, nor to its advantageous action in overcoming the effects of selective fading, since it is equally valuable when the receiver is adjusted for the standard broadcast band of 550 to 1700 kilocycles (kc).

Considering the circuit diagram in detail, the numeral l designates a pentode type tube which is employed in the intermediate frequency (I. F.) amplifier network. It will be understood that the I. F. input transformer 2 has its resonant primary and secondary circuits each tuned to the operating I. F. value. The primary circuit of transformer 2 will be coupled either tp the output circuit of the converter, or to the plate circuit of a prior I. F. amplifier stage. The I. F. transformer 3 has its primary and secondary circuits each tuned to the operating I. F. value. The I. F. value may be chosen from any range of common.. ly used frequency values. As isvwell known to those skilled inthe art, the demodulator, or

,second detector, may comprise a diode whose anode and cathode are respectively connected to the opposite sides of the resonant input circuit 4.

The demodulator diode is shown as having its electrodes embodied in the envelope of a tube 5 which may be one of the duo-diode-triode type.

Vresistor 25.

One of the diodes is employed as the demodulator, while the other diode is employed as the auto-matic volume control (AVC) rectifier. The numeral 6 designates the demodulator load resistor which is bypassed by condenser 1 for I. F. currents. The two diode anodes are coupled together by an I. F. bypass condenser. The AVC diode anode 3 is connected to a source of negative bias through resistors l9v and I0. The cathode of tube 5 is grounded. The lead II is connected to an intermediate point on load resistor 6 so as to derive therefrom a unidirectional voltage which corresponds to rectified carrier component.Y

The unidirectional voltage may be utilized for operating a tuning indicator tube of any known type.

From the same intermediate point on load re- I sistor` 6 audio voltage is taken' off, and .is fed/to a tone-compensated volume control. The latter is shown as comprising the resistor-condenser network enclosed within the dotted rectangle I2. This type of volume control device is well known tothosev skilled'in the; art, and is not a part of th'eipresent invention. Hence, it is sufficient for the purpose of this application to point out that itsjgeneral function` is to provide tone compensation at different settings of the volume control tapgI3. When the tap I3 is aty point I4, or below, there lis secured low Vaudio frequency compensation, `and the audio response resulting therefrom is dish-shaped. Whenfthetap I3 is above point I4the audio response characteristic is convex.

The tap I3 `is connected to ground through a path comprising condenser I5 and resistor I6 connected-in series. from the upper end of resistor I6, and is fed to the control gridV of audio amplifier tube I'I through `a path which-comprises lead I8, resistor I9 and lead`20.- The cathode of tube II is grounded, while-the plate thereof is connected to a point of Ipositive potential on the voltage-supply source of-thereceiver.: A manual tone control means maybe provided-for the-plate circuit of audio Varnpliflerl'! `by connecting condenser 2| and variable resistor 22 between the plate of tube I'I and ground. The adjustment of the tap of' variable resistor 22 willfvary the bypassing of the higher audiofrequenoies from the plate circuit of tube I1. The condenser 23 .transmits audio frequency energyfrom theY plate circuit of tube l1 to the input grid-of onelof a pair of push-pull connected amplifier tubes. `The grid of tube 24 is connected to a source of negative biasing potential by a The other tube of the pair of push-pull tubes is designated yby 26. The plates of the tubes 24 and 26 are arranged in push-pull relation,-and are connected to opposite ends of the primary windingroffaudio output transformer 21. The cathodes of tubes 24 and 26 are-shown grounded. Thecontrol grid of tube 26 is connected by the audio coupling condenser 28 to the plate Z9 of ,the triode-section of tube 5. The triode section `comprises the common cathode 3D, the control grid 3I and the plate 29.y The cathode 3U is, of course,4the cathode of each-of the demodulator and AVC rectifier diodes.

'Ifhev triode 3Il-3i-29 functions asa phase inverter' device.- The control grid 3l is maintainedat a negativepotential with respect to the grounded cathode 3l) by connecting the grid `to a source of negative biasing potential through a path comprising lead 32, audio lter resistor 33 and resistor 34. The control grid I'I of audio ,amplifier tube I'I secures its negative bias from Audio voltage is taken off the same resistor`34 through a path comprising lead 20, resistorlg and grid leak resistor 35.

It is pointed out that resistor I9 functions to provide an impedance to develop the feedback voltage thereacross when the volume control arm I3 is set either near maximum or minimum. At these settings the volume control impedance is low, and. the action of the automatic tone control would not be satisfactory without resistor I9. The condenser 36 is connected to ground from the junction of resistors 35 and 34, and acts to bypass hum frequencies. The plate 29 of the phase inverter section is connected -to a source of positive potential through a resistor 40. Audio voltage is fed to the control grid 3I of the phase inverter section Yfrom an intermediate point on the grid circuit resistor 25 by condenser 4I. By virtue of the phase inversion action of the triode section of tube 5 the phase of the audio voltage at the grid circuit of tube 24 is reversed in polarity.v Hence, theaudio voltage fed to the singleended audiostage I1 lmay be amplified, and transmitted to the input circuits of the push-pull audio amplir tubes 24 yand 25.

In order to provide a control over the higher audiov frequencyresponse-of the audio network, there is provided a. tone control tube having a cathode 50. While this tube is shown as of the pentode type, it is to be clearly understood that it may be of any other type, as for example, a triode. The cathode 50 is shown connected to ground. The control grid 5I is connected to the anode end of the AVC'load resistor 9 through a path which comprises a resistor 52 of high magnitude, the auxiliary AVC lead 53 and a filter resistor. The junction of the load resistors 9 and I0 is connected by the main AVC lead 55 and filter resistor 56 to the low potential side of the secondary circuit of I. F. transformer 2. It will be noted that AVC rectification is of the delayed type. In other words, there will be no flow of rectified current through resistors 9 and Ill untilthe initial negative bias on diode anode 8 is overcome.

The AVC lead 53, which may include one or more filter resistors for preventing the flow of pulsating current components, may be connected to the control grid of the rst detector, if desired. The lower end of resistor 52 is connected to ground through a lter condenser 63. The resistor-condenser network 52-60 functions as a filter to remove pulsation components. The control grid 5I has applied to it audio voltage fromthe plate circuit of the phase inverter section. This audio voltage is applied through a path'comprisng lead 6I, condenser 62. and resistor 63. The series path 62--53 has a low impedance to the higher audio frequencies, but has a high impedance to the lower audio frequencies. The plate and screen of the tone control tube are connected through a high resistor B4, and the screen end of resistor 64 is connected to a point of proper positive potential of the power supply network.

The plate 'Ill of the tone control tube is connected to the control grid I1 of the audio conplifier tube I1 through a path which comprises condenser .having a low impedance for the higher audio frequencies. but of a high impedance to the lower audio freouencies. For example, the condenser 8l! may have a magnitude of the order of A82I micro-microfarads. It is pointed out that the higher audio frequency components of the audio voltage fed to the control grid I are degeneratively applied through condenser 8U to the control grid I7 after amplication. It will now be clear that the AVC connection to control grid 5I provides a means for correlating the degree of high audio frequency degeneration with carrier amplitude.

Assume that the received modulated carrier energy is of relatively low amplitude, as when receiving distant signals or weak stations, then the AVC bias will be of minimum value. In that case the tone control tube will have a maximum gain, with the result that the degeneration of the higher audio frequencies at grid I'l will be a maximum. In this way the higher audio frequencies will be substantially limited thereby preventing the reproduction of undesired noises which are rnost prevalent in the higher portion of the audio spectrum. Conversely, when the received .modulated carrier energy is of high amplitude the AVC bias will greatly-reduce the gain of the tone control tube thereby minimizing the higher audio frequency degeneration, and permitting a higher fidelity of reproduction.

There is provided the auxiliary degenerating condenser 99 between the plate ID and the control grid 5l. The degenerating condenser functions to stabilize the control tube, and prevents audio self-oscillation effects which might otherwise arise. It is pointed out that by virtue of the delayed bias applied to anode 8 of the AVC rectifier, the degeneration of the higher audio frequency components will remain a maximum until the received carrier amplitude attains a predetermined intensity. This predetermined intensity may be chosen according to the needs of the particular locality where the receiver is located. As soon as the received carri-er intensity exceeds this predetermined carrier intensity the degeneration of the higher audio frequencies is reduced sharply by virtue of the nature of the circuit which is employed in the tone control circuit. It is, also, pointed out thatl should selective fading commence, the higher audio frequency degeneration will tend to reduce the noises which generally accompany the fading of the carrier with respect to the modulation side bands.

Whilel I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modications may be made without departing from the scope ofY my invention, as set forth in the appended claims.

What I claim is:

1. In combination in an audio frequency amplifier circuit. a first amplifier tube having input and output terminals. means for applying audio energy to be amplified to said input terminals, a push-pull audio ampli-der having an input circuit. a first connection' from the output terminals of said first tube to said input circuit. phase inverter coupling said first tube output terminals to said input circuit. and means functioning as a degenerative feedback path for the higher audio frequency components, said path being connected between the output of said inverter and the input terminals ofsaid first tube.

2. In a system defined in claim l. said degenerative path comprising a tube functioning to amplify said components, and means for regulating the gain of said tube.

3. In a system as defined in claim 1, a tube provided in said degenerative path, an auxiliary feedback path between the input and output electrodes of said last mentioned tube for prevention of audio oscillations. .i

4. A method of controlling the response of an audio frequency transmission network which comprises feeding audio frequency energy thereto, separating the higher audio frequency components from the energy, subjecting the separated components to amplification, providing supplemental degeneration during amplification of the components to prevent audio oscillation, applying the amplified components to the audio network in degenerative phase, and controlling the amplification of said components inversely 5. In combination in an audio frequency amplifier circuit, a first amplifier tube having input and output terminals, adjustable means for applying audio energy of a predetermined intensity to said input terminals, push-pull connected audio amplifier tubes having a common input circuit, a first connection from the output terminals of said first tube to said input circuit, a phase inverter tube coupling said first tube output terminals to said input circuit, a degenerative feedback path for the .higher audio frequency components connected between the output of said inverter and the input terminals of said first tube, said degenerative path comprising a tube functioning to amplify said components.

6. In a system as defined in claim 5, an auxiliary feedback path between the input and output electrodesof said degenerative tube for prevention of audio oscillations.

7. In combination, a demodulator having an audio modulated carrier input circuit, a first..

audio amplifier tube having input and output terminals, means for applying audio frequency energy from the demodulator to said input terminals, a push-pull audio amplifier having a common input circuit, a first connection from the output terminals of said first tube to said input circuit, a phase inverter coupling said first tube output terminals to said input circuit, means functioning as a degenerative feedback path for the higher audio frequency components, said path being connected between the output of said inverter and the input terminals of said first tube, and means for regulating the gain of said path in response to carrier intensity variations.

8. In combination in an audio frequency ainplier circuit, a first audio amplifier tube having input and output terminals, means for applying audio frequency energy to be amplified to said input terminals, an audio utilizing network having an input circuit, a rst connection from the output terminals of said first tube to saidinput circuit, `a phase inverter coupling said first tube output terminals to said input circuit, a path of low impedance for the higher audio frequency components, said path being connected between the output of said inverter and the input terminals of said first tube for applying the said components in degenerative phase to the latter.

9.In a system as dened in claim 8, a tube provided in said degenerative path, an auxiliary negative feedback path between the input and output electrodes of said last mentioned tube for prevention of audio oscillations. 

